The relevant section in the FIA Formula 1 2012 Sporting Regulations say:
20.2 Drivers must use the track at all times. For the avoidance of doubt the white lines defining the track edges are considered to be part of the track but the kerbs are not. A driver will be judged to have left the track if no part of the car remains in contact with the track. Should a car leave the track the driver may rejoin, however, this may only be done when it is safe to do so and without gaining any advantage.
The evidence, such as it is, turns out to be pretty clear cut:
There we see Mr. Vettel well off the left hand side of the track as denoted by the white lines. In this case, the blue-and-white kerbing is not part of the track.
But -- and this to my eyes is the most interesting part -- the McLaren of Mr. Button is hanging off the track. Mr. Button has clearly not left the track, since his right hand wheels are still between the white lines.
Here's why this is interesting: had Mr. Vettel kept his car between the white lines -- or just tried to hang on to the track with this right had wheels -- the two cars would have collided. The question as to who would have been "responsible" for the collision would be a toss up based on which result the view would most like to see. Button's supporters could argue that since he had the inside line around the hairpin, his car had "the racing line" and therefore Vettel should have yielded. On the other hand, since Vettel's car was on the outside, it had the racing line once the line came out to him, and Button should have yielded.
Personally, I would rather Button have the place since he is further behind Alonso in the Driver's Championship table; but I also personally see the value in drivers given the option to avoid collisions and surviving to contest another corner, so I would have ruled in Vettel's favor.
I think if the FIA is going to enforce this "leaving the track" regulation in these circumstances they are going to have to be clearer about who has to yield in these circumstances, because otherwise drivers are going to hang onto the track and we are going to have collisions as cars race.
A prime example of this would be the collision in Valencia between Maldonado and Hamilton. Yes, one could argue that Maldonado was off track immediately before the collision; however we could equally argue that Hamilton pushed him there. In any event, the stewards evidently considered Hamilton's retirement punishment enough; and Maldonado was awarded a drive-through penalty for being there at the time.
The FIA really needs to figure this one out. If drivers end up retiring after collisions caused by desperately hanging on to the track to avoid this "leaving the track" nonsense -- how is that a net benefit for F1?